Metal dome switch is a thin switch widely used on mobile communication devices, input devices of computer systems or IC cards. It usually contains a thin and arched metal blade and can provide metallic characteristics such as electric conductivity and elasticity.
During manufacturing processes to install the metal dome switch on a printed circuit board (PCB in short hereinafter), air often is trapped in the metal dome switch. The trapped air makes depression of the switch difficult and click feeling suffered. Many techniques have been proposed to expel the air trapped in the metal dome switch. References can be found in U.S. Pat. Nos. 5,901,834, 6,441,330, 6,531,671, 6,917,007, 6,982,394 and 7,145,092. They mostly have an opening or a conduit formed on the bounding surface between the metal dome switch and the PCB to allow the air to flow out, so that depression quality of the metal dome switch is not affected by the air trapped inside.
However, the conventional techniques mentioned above cannot be used on IC cards embedded with a metal dome switch. FIGS. 1A and 1B illustrate a conventional IC card 1. It has a metal dome switch 2 located on a PCB 3 corresponding to electrodes located thereon. The PCB 3 is evenly coated with an adhesive tape 4 to bond the metal dome switch 2. The electrodes include a peripheral electrode 5 and a trigger electrode 6 located in the peripheral electrode 5. The metal dome switch 2 has the periphery in contact with the peripheral electrode 5 in regular conditions. When the metal dome switch 2 is depressed, the center portion thereof is moved downwards until touching the trigger electrode 6 as shown in FIG. 1B, then a conductive connection is formed between the peripheral electrode 5 and the trigger electrode 6 through the metal dome switch 2 to generate an electric signal. When the depressing force is released, the metal dome switch 2 bounces back to its original position due to metal elasticity, then the connection between the peripheral electrode 5 and trigger electrode 6 is broken. As the metal dome switch 2 and the PCB 3 are integrally sealed in the IC card 1, air also is trapped between the metal dome switch 2 and the PCB 3. Depressing the metal dome switch 2 forcefully, the air will spill to the periphery where the adhesive tape 4 is located, or even pass through the adhesive tape 4, and an air bubble 7 or a gap is formed in the IC card 1 as shown in FIG. 1B. As a result, a packaging and sealing defect occurs. This will cause disintegration of the structure of the IC card 1, and also impact normal function of the PCB 3.